These teachings relate generally to detecting concealed objects, and, more particularly, to detecting concealed objects utilizing radiation in a predetermined range of wavelengths.
An infrared camera can see the environment based on temperature difference between neighbor elements of the image. For example, cold object on the warm background could be seen on the monitor connected to the infrared camera as black spot on the white background. An Object under cloth also could be visible if object is cold enough compared to the body temperature. If the object under cloth is in contact with the cloth for a sufficiently long period of time, the body transfers heat to the cloth and the object and eventually the object will be almost indistinguishable (invisible) or more accurately, the object is losing its visibility or contrast in the infrared. The less sensitive an infrared device, the faster the object loses its visibility.
However, temperature between the object and cloth background will never be theoretically equal because of different thermal conductivity of an object and cloth. A temperature difference exists even the object on the body is located for any length of time. This difference could be negligible for today's devices to reveal such objects under cloth. For example, advanced cameras with thermal resolution not better than 20 mK cannot see real objects under the cloth if the object is in contact with the body for about 10 minutes or more.